Saving electric power has been one of the key improvements in electric equipment in recent years, often in consideration of environmental issues. This trend is particularly obvious in electric appliances powered by batteries. General means for achieving power savings include cutting back on waste of electric power consumed by an electric machine and increasing efficiency of a power supply source of the electric machine. In one example, when an electric machine is in a non-operative state, the machine is held in a standby state to stop the operations of circuits in the machine so as to reduce power consumption. When, however, the power supply source itself has low efficiency, a sufficient power savings cannot be expected.
Switching regulators and series regulators are common electric circuits used as power supply apparatuses. The switching regulator generally has a relatively high efficiency at rated load. On the other hand, it has relatively large output voltage ripples and produces noise in operation, and its internal power consumption becomes relatively large. Therefore, when supplying a power to a light load that consumes a relatively light current, the switching regulator has dramatically reduced efficiency. Moreover, the switching regulator has relatively low output voltage stability since it is relatively slow in raising output voltage and in responding to variations in input voltage and to load fluctuation.
The series regulator has a relatively low efficiency due to a relatively large power consumption of an output control transistor when supplying electric power to a heavy load that consumes a relatively large current, but has less output voltage ripple and produces relatively little noise in operation. In addition, the series regulator allows reduction of internal power consumption of the power supply control circuit itself. Therefore, some series regulators are more efficient than a switching regulator when the load is relatively small. Furthermore, the series regulator can easily raise the output voltage and quickly respond to variations in input voltage and to load fluctuation. In addition, the series regulator has relatively high output voltage stability.
As an example, Japanese Laid-Open Patent Application Publication No. 2001-197731 describes a power supply apparatus including both a switching regulator and a series regulator. This power supply apparatus activates one of the regulators depending on load current in order to increase power supply circuit efficiency.
FIG. 1 shows a schematic circuit diagram of a DC-to-DC converter 66, an example of a power supply apparatus described in the above Publication No. 2001-197731. In FIG. 1, the DC-to-DC converter 66 includes a series power supply (SPS) circuit 100 and a switching power supply circuit 102. The series power supply circuit 100 has a nearly constant electric power conversion efficiency of approximately 70%, regardless of the load current. The switching power supply circuit 102 provides efficiency greater than 80% at a relatively large load current while providing reduced efficiency as the load current becomes smaller. That is, this DC-to-DC converter 66 activates the series power supply circuit 100 for a light load and the switching power supply circuit 102 for a heavy load.
Each of the series power supply circuit 100 and a PWM (pulse width modulation) controller 108 included in the switching power supply circuit 102 has an enable (EN) terminal. When the enable terminal of one of the circuits is in a low state and is activated, the corresponding power supply circuit is caused to output a predetermined voltage. In other words, at a heavy load, the switching power supply circuit 102 is activated and, at the same time, the series power supply circuit 100 is inactivated by changing a standby signal input to an input terminal 109 to a low state. On the other hand, at a light load, the standby signal is changed to a high state to stop the operations of the switching power supply circuit 102 and to activate the series power supply circuit 100. In this way, at a light load, the series power supply circuit 100 is used in place of the switching power supply circuit 102, which has reduced efficiency at a light load. Therefore, the overall efficiency of the DC-to-DC converter 66 is increased.
However, the DC-to-DC converter 66 is required to have a switching circuit 116 to switch between the series power supply circuit 100 and the switching power supply circuit 102 and also an enable terminal for each of the series power supply circuit 100 and the PWM controller 108 of the switching power supply circuit 102. This makes the circuit of the DC-to-DC converter 66 more complex and accordingly increases manufacturing cost. Furthermore, when the standby signal is changed from the low state to the high state, the switching power supply circuit 102 would immediately lower its output voltage but the series power supply circuit 100 may delay in raising the output voltage to a predetermined level. Therefore, an output voltage at a common output terminal may momentarily drop, a problem referred to as an undershoot.
It would be advantageous to have improved power supply techniques that are efficient yet avoid problems such as undershoot.